1. Field of Invention
The present invention relates to flow meters and more particularly to improved current collection means in an ion flow meter.
2. Description of the Prior Art
In many devices involving the movement of a fluid, means for measuring flow characteristics such as the mass flow, flux and velocity of the fluid are required. A device of this type is disclosed in U.S. Pat. No. 3,470,741 entitled Mass Flow Meter Apparatus issued on Oct. 7, 1969 to E. J. Durbin. The apparatus is described in terms of a mass flow meter and its overall operation is straightforward. The fluid of interest is passed through the meter which includes ionizing means proximate to the path of fluid flow and ion collecting means also proximate to the path of fluid flow and opposite to the ionizing means. The operation of such apparatus includes energizing the ionizing means to provide a source of ions at the boundary of the stream of fluid. As the fluid moves along a flow axis, the ions migrate across the flow stream in a direction essentially perpendicular to the flow axis as a result of an applied electric field and are collected on an ion receiving electrode means which is a distributed collector resistor comprising a nonconductive substrate such as glass and a resistive member such as wound wire or a resistive coating on the substrate. The ions introduced into the stream from the ion source have a spatial and energy distribution with average properties which can be represented by the median ion. During the ion migration across the stream, the motion of the fluid displaces the position of the median ion in the direction of flow in an amount which is exactly proportional to the flow velocity of the stream, assuming fluid particle density is constant. Alternatively the displacement is proportional to flux assuming that both the flow velocity and density are variable. The position on the ion receiving means at which the median ion is collected is established by measuring the electric currents passing from the upstream and downstream terminations of the resistive member and this median ion displacement information is converted to velocity or flux.
As a practical matter, the accuracy of the device in a normal working environment is often compromised due to the formation of a nonuniform coating on the distributed collector resistor. The coating so formed causes the current in the collector due to ion migration across the fluid transverse to the flow direction to be sensed in a manner which is unrelated to the flow of the fluid. The performance of such apparatus could be improved significantly if the effective electricl resistance of the ion collector means could be desensitized with respect to the buildup of nonuniform coatings.